Robots aren't known for being light on their feet, but a new artificial muscle that can be powered by a sunbeam could change that. Researchers have developed a photoresponsive actuator - a gel-like material that bends when it is exposed to light. Akira Harada at Osaka University in Japan and colleagues have developed a prototype that works in water. When hit with UV light at a wavelength of 365 nanometres it expands, bending and increasing in mass by taking on water. Visible light at a wavelength of 430 nm restores the muscle to its previous form. "The gel absorbs water like an expanding and contracting sponge," says Harada.